Variable reactance device for coaxial lines



JuneA l0, 1947. f '0. M. wooDwARD, JR 2,422,160

' VARIABLE REACTANCE DEVICE FOR COAXIAL LINES Filed June 13, 1944 A27/'575% M45 4/ 47 4f I 57| l 49;

l. l '44 HP1-*I INVENTOR. gl/lwdwarf HTTIFMEY Patented June 10, 1947 VARIABLE REACTANCE DEVICE FOR COAXIAL LINES Oakley M. Woodward, Jr., Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 13, 1944, Serial No. 540,108

4 Claims.

This invention relates to variable or adjustable reactance devices of the type used in radio frequency transmission line circuits for impedance matching and the like.

The principal object of the instant invention is to provide an improved variable reactance stub structure capable of withstanding severe vibration and exposure to weather conditions, as encountered in aircraft antenna installations.

Another object is to provide a device of the described type which is readily adjustable, but does not include sliding ngers or contacts.

A further object is to provide an improved variable reactance stub whichmay be locked mechanically secure and watertight at any required setting.

These and other objects will become apparent :to those skilled in the art upon consideration of the following description, with reference to the accompanying drawing, wherein Figure 1 is a sectional elevation of a variable reactance stub in accordance with the present invention; Figure 2 is a view in elevation of a pair of reactance stubs similar to that shown in Figure 1, in operative position upon a coaxial transmission line; Figure 3 is an outline drawing indicating the electrical connections of the system of Figure 2; and Figure 4 is a sectional view of a typical prior art reactance stub structure.

The term reactance stub as used herein is dened as a transmission line section connected at one of its ends to thepoints across which reactance is to be inserted, and open-circuited or short-circuited at its other end. It is well known that such transmission line sections, r stubs, will exhibit the properties of reactances which may be made of substantially any required value by proper adjustment of the eilective stub length. Reactance stubs find application particularly at high frequencies, since lumped pure reactance elements for high frequencies are difficult or impossible to construct, and stubs for high frequency use are of reasonably small dimensions and relatively easy to make.

Referring to Figure 4, an inductive stub typical of prior art practice includes an outer cylindrical conductor I connected to one of the terminals across which inductive reactance is desired; for example, the outer conductor 3 of a coaxial transmission line 4. An inner conductor 5 is disposed coaxially within the conductor I and connected at one end to the inner conductor 8 of the coaxial line 4. A conductive plug 'I is provided within the conductor I, for short-circuiting the free end of the stub. 'I'he plug 'l is adjustable in position lengthwise of the stub for adjusting the magnitude of the reactance across the line 4.` If repeated adjustment is not required, the plug 'I is soldered or otherwise permanently secured in position. Otherwise, spring contact fingers 9 are provided for ensuring electrical connection of the piug l to the members I and 5. The outer conductor I may be provided with a longitudinal slot for introduction of a tool, such as a screw driver, for adjusting the plug 1.

The above-described type of reactance ystub structure is widely used, notwithstanding the difficulties in making and maintaining accurate adjustment of the position of the shorting plug, the undesirable characteristics of spring contact g fingers, and the susceptibility to the entrance of moisture, dirt, etc.

The present invention contemplates the use'of an inductive stub, effectively in series with a capacitance, and coupled to the point of utilization through a concentric line section. .The inductive stub is varied in length by adjustment of the inner conductor, rather than the shorting plug. This adjustment simultaneously varies the length of the coupling line and the capacitance in series with the stub. The use of three simultaneously variable elements, rather than the single variable of stub length, enables the designer to select, dimensions such that the variation of reactance as a suction of adjustment position is substantially linear over a wider range than is possible with the usual short-circuited adjustable stub.

Refer to Figure 1. An outer conductor Il is connected at one end I3 to one side of the circuit where reactance is required, for example, the outer conductor I5 of a coaxial transmission line Il'. The conductor I\I is preferably made one-quarter wavelength long at the frequency at which the system is to operate, although other lengths may be used, for example, three-quarter Wavelength. The other end of the conductor Il is closed with a plug I9, threaded or soldered or otherwise permanently secured to the conductor II. An inner conductor 2l is connected at one end to the other side of the utilization circuit, in this case the inner conductor 23 of the line I1, and is open-circuited at its other end 25.

A tubular conductor 21 extends through, and is in threaded engagement with, an aperture in the plug I 9. The conductor 21 is coaxial with the conductor 2 I, and may be adjusted longitudinally with respect thereto by rotation in the plug I9. A knurled adjusting knob 29 is provided at the end of the conductor 21. A washer 3| and a lock nut 33 are provided for iixing the adjustment of the conductor 21 and preventing the entrance of vwater through the screw threads in the plug I9.

The operation of the above-described device is as follows:

The outer conductor and the tubular conductor 21, with the plug I9, cooperate to function as a sh'ortecir'cuited. stub, presenting re'actance between they lower end 35 of the conductor 21 and the outer conductor 1||.V Since this stub is shorter than one-quarter wavelength, the I'`Y The inner conductor. 2|Y

actance is inductive. and the tubular conductor 21 act as an 'open-cir-Lv nected atY one of its ends to said series-connected 1 sections, and means for adjusting the longitucuited stub presenting capacitive reactance be'- tween the lower end 35 of the conductor 2l and.

the adjacent point 3l on the inner conductor 21'..

This capacitive reactance is electrically in series with the inductive reactance of the snorted stub. The outer conductor I and the inner conductor 2|, from the point 31 to the point 23, constitute' a section of coaxial line. of length 0. The net reactanee between the point 31 and the outer condnctor' Il is transformed as av function of the length 9 to another value of reactance, which appears between the point 23 and the outer conductor i of the line l'l. is positive i. e. inductive) for any position of the conductor 21, if the outer conductor H is onequarter wavelength.. Any positive reactance from zero tof substantially infini-te value may be obtained across the line byadjustment of the conductor 2T.

Referring. to Figures, 2 Vand two devices 4| and-43. of the type illustrated in FigureV 1 are connecte-d to a coaxial line. 319 at points respectively one-quarter and one-half wavelength distant from a junction point 45. Coaxial lines 41 The. latter reactance such as antennas, not shown. The other end of the line 39 extends to a radio transmitter or receiver or other load,.not shown. The parallel connection of the lines. 4T: and 49. to the point produces aV standing wave having` a low voltage point at 45 and a highY voltage point at 5|, onequarterv waveA away. By proper adjustment of the two reactance devices 4|. and. 43., the feed line to.- thetransmitter may be matched, preventing,V the. appearance ofstanding waves thereon. fir-mismatch of the lines 41 .and` i9 caused by the installation. of. the feed system may be corrected alsoebyadjustmentof the; stubs M andlw` Thel invention hasbeen described. asv an improved` variable-reactance.-device, comprising concentric, conductors arranged to. act as, an` inductive stubfin series. with a capacitor and. coupled to: the utilization point throughv a line impedance trans-former. Reactance devices maybe` readily designedineaccorda-nce withthe invention to have the advantages `of. greater stability and ease of adjustmentthan. similar prior art devices.

dinal position of said integral conductors to adjust simultaneously the lengths of said three line sections.

. 2;. Tieinvention as set forth in claim 1, wherein the sum ofthe lengths of said short-circuited section' 4and saidV third section is substantially one-'quarter wavelength at operating frequency.

3. A variable reactance device comprising an outer tubular conductivemember substantially one-duarternwavelengtn lon-g at operating frequency, Va second tubular conductive member extending coaxially within said'outer member and connected thereto at one end. ot said outer memhere` and an` innermost conductor extending Icoaxially with-in said` outer conductor. and into said second conductor, said second conductor being adjustable.y longitudinally with respect to said cil-rter.and. innermost conductors. Y v

e; A variable reactance device including atufbular outer conductor substantially anintegral odd,r number of quarter wavelengths long, a second tubular conductor connected; to one. end of saidl first conductor` and extending c-oaxially within said outer conductor by an adjustable amount, and an innermost conductor extending coaxially within saidr second conductorV and spaced radially therefrom, whereby said firstr and second-con ductors; cooperateV to function as.Y an inductive shortfcircuited. concentric line stub, said second and 'third' conductors4 cooperate to function as a c-:apaciftive4 open-circuited concentric line stubl in series w-itn said short-circuited stub, and. said firstgand third conductors cooperate to'y function asa concentric. line. transformer' section con-necteditobsafidseries-cormected stubs. Y Y

- OAKLEY M. WOODWARD, JR.

REFERENCES CITED Naimber' Name Date 2,1r59g648i' Mayl'233, 1=939 218:1?,3931 Linden'blad DBC'. 5,. 1939 

